Fiber Bragg grating sensors (FBG sensors) have attracted considerable attention in sensing temperature and strain on an optical fiber due to the variation in the spectral response of the grating as a result of strain and temperature on the grating structure. These FBG sensors offer important advantages such as electrically passive operation, EMI immunity, high sensitivity and multiplexing capabilities. Fiber gratings are simple, intrinsic sensing elements which traditionally have been UV photo-inscribed into photosensitive Ge-doped silica fiber. Each FBG sensor has a characteristic retro-reflective Bragg resonance or Bragg wavelength, which is dependent upon the periodicity of the grating within the fiber and the effective refractive index of the fiber. The FBG sensors can then easily be multiplexed in a serial fashion along a length of single fiber. When embedded into composite materials, fibers with an array of FBG sensors allow for distributed measurements of load, strain, temperature and vibration of the material creating what has is commonly referred to as “smart structures” where the health and integrity of the structure is monitored on a real-time basis.
Typically fiber Bragg gratings are generated by exposing the UV-photosensitive core of a germanium doped silica core optical fiber to a spatially modulated UV laser beam in order to create permanent refractive index changes in the fiber core. Such a spatially modulated UV beam can be created by using a two-beam interference technique as disclosed in U.S. Pat. No. 4,807,950 by Glenn et al. or by using a phase mask as disclosed in U.S. Pat. No. 5,367,588 by Hill et al.
A limitation of the prior-art UV-induced fiber Bragg gratings, especially for high temperature sensor applications is that operation of the sensor at elevated temperatures results in the erasure or annealing of the UV-induced color centers which are responsible for the induced index change of the grating. In fact, at temperatures approaching the glass transition temperature of the fiber, which for silica is approximately 1000° C., total erasure of the induced index modulation results. The fiber also is modified at such high temperatures making it brittle with diffusion of the core material into the cladding. The fiber can easily be deformed by its own weight.
Another method for creating permanent photoretractive index changes in glasses employs the use of intense UV beams with fluences or energy/unit-area per laser pulse densities that approach those required to produce macroscopic damage of the glass. Askins et al. in U.S. Pat. No. 5,400,422 teach a method for producing permanent photoretractive index changes in the photosensitive cores of Ge-doped optical fibers with single high intensity UV laser pulses. The high intensity portions of the interference fringes created by two crossed UV beams split from a single UV beam create localized damage at the core-cladding interface within the fiber. Because the process for inducing index change is one of structural change due to localized physical damage to the glass, rather than due to UV photoinduced color center formation, the induced index change is more robust and does not decrease with elevated temperature. In fact Askins et al. disclose that gratings produced in this way cannot be removed by annealing until the fiber or waveguide approaches the material's glass transition temperature. The drawback of this approach for induction of index change is that the Bragg gratings produced in this fashion have relatively low refractive index modulations (Δn=10−4) and are mechanically weak since the effective refractive index change results from periodic localized damage at the core-cladding interface. When the pulse duration is long (>a few tens of picoseconds) laser-excited electrons can transfer energy to the surrounding lattice faster than the thermal diffusion of the material can remove the energy resulting in damage. If the laser pulse continues to feed energy into the damage site, the damage can propagate beyond the irradiated zone. For damage grating structures written with long laser pulse durations greater a few tens of picoseconds, the spectral quality of the resulting Bragg grating is often poor.
The prior art FBG sensors suffer from serious limitations when measurement of displacement, temperature, strain and pressure are required at high temperatures. The materials used to fabricate the FBG sensing element deform or melt. The softening or glass transition temperature of silica optical fibers is typically 1000° C. At temperatures equal to or above this, silica optical fibers are not useful, even if they are coated with materials which melt at higher temperatures.
One approach to fiber-based measurements at high temperatures is to use sensor elements fabricated in fibers made of sapphire. Because sapphire has a very high glass transition temperature (˜2030° C.), a sensor fabricated in this fiber can be operated in high temperature environments. Currently, sapphire fibers are only made in the form of rods with diameters as low as 50 μm. These rods lack a cladding or a coating material similar to conventional silica fibers. The large diameter of the sapphire fiber does not support single mode propagation at typical wavelengths used for FBG sensors in silica fiber thus does not allow the implementation of the FBG sensor as described previously. A technique for the fabrication of a sapphire based optical fiber interferometer based on the fabrication of a Fabry-Perot etalon on the tip of the sapphire fiber has been taught by Murphy et al. in U.S. Pat. No. 5,381,229. Although this device is effective as a point sensor, is relies on the monitoring of the broadband interference fringe pattern generated by the Fabry-Perot etalon and therefore is extremely difficult to address in a wavelength-division or time-division multiplexing fashion. This makes the Fabry-Perot based fiber sensor inappropriate for distributed sensor arrays.
Mihailov et al. in U.S. Patent Application 20040184731 published Sep. 23, 2004, incorporated herein by reference discloses a technique for fabrication of Bragg grating structures in optical media such as optical fibers and waveguides with an ultrafast (<500 ps) laser source and a phase mask using a direct writing technique. The resultant grating structures have high induced-index modulations (>1×10−3). Since the refractive index change need not be dependent on the dopant in the core or cladding of the optical fiber or waveguide, refractive index changes can be induced in both regions of the waveguide. Processes that employ high-intensity laser pulses in the femtosecond pulse duration regime for creating permanent changes in the refractive indices of glasses have been explored by several groups of researchers. K. M. Davis et al. disclose a technique for inducing index change in bulk glasses with ultra-high peak power femtosecond infra-red radiation in Opt. Lett 21, 1729 (1996). The creation of waveguides in bulk glasses using this technique is taught by Miura et al. in U.S. Pat. No. 5,978,538 while the modification or trimming of existing waveguide structures is taught by Dugan et al. in U.S. Pat. No. 6,628,877. The physical process that causes the refractive index change in the materials is due to the creation of free electrons through non-linear absorption and multi-photon ionization of bound charges, followed by avalanche ionization and localized dielectric breakdown as these free electrons are accelerated by the intense but short time duration laser field. Also, this leads to a localized melting and restructuring of the material and a concurrent increase in the index of refraction. Recently Sudrie et al. in Opt. Comm., vol. 191, no. 3-6, pp. 333-339, 2001 disclosed a technique for inducing index changes in bulk silica using a femtosecond IR laser source with a power threshold below that needed for multi-photon ionization. The resultant index change induced by multiphoton absorption produced microscopic defects or color centers in the lattice. Unlike the index change created through localized dielectric breakdown of the material, the color center induced index change can be removed or annealed out at temperatures below the glass transition temperature of the material.
For a fiber grating sensor application it is desirable to obtain a single mode response in the reflection spectrum because the bandwidth of the response is narrower and there is improved signal to noise ratio as there is only one mode being inspected rather than a superposition of hundreds if not thousands of modes that can be supported in a multimode fiber. For step index fibers the normalized frequency or V number is given by:
                    V        =                                            2              ⁢              π              ⁢                                                          ⁢              r                        λ                    ⁢                                    (                                                n                  co                  2                                -                                  n                  cl                  2                                            )                                                          (        1        )            
where r is the core radius, l is the wavelength and nco and ncl are the refractive indices of the core and cladding respectively. For single mode operation, V≦2.405. When a fiber is tapered, by using the hydrogen flame brushing technique for example (see Bilodeau et al U.S. Pat. No. 4,895,423 enclosed herein as reference) the ratio of cladding/core radii remains constant however V decreases. As disclosed in J. D. Love et al IEE Proceedings Journal vol. 138, no. 5, p.343-354 (1991), herein enclosed as reference, when single mode optical fiber is tapered down such that the normalized frequency or V number of the taper is V<0.84, the fundamental LP01 mode is no longer confined to the core but instead is guided by the cladding-air interface resulting in a mode field with the same diameter as the tapered fiber. By launching this expanded fundamental LP01 mode that has the same mode field diameter as the cladding of the multimode fiber, only the fundamental mode of the multimode fiber will be excited, in the absence of perturbations such as twisting and bending of the multimode fiber. The retro-reflection from the grating would still consist of a small number of modes however the fundamental from the grating would be reciprocally collected by the taper and converted to a core guided LP01 as it exited the tapered region of the coupling fiber.
As the technology for fabrication of long lengths of single crystal fiber is in its infancy, it is difficult and costly to have long lengths (>4 m) of sapphire fiber for a multiplexed sensor web. An alternative to single strands of fiber may be to couple short lengths of sapphire fiber, with gratings present, to single mode fiber which has been appropriately tapered to excite the fundamental mode of the sapphire fiber segment. If an identical taper to the input taper is placed at the output of the multimode sapphire fiber, then a single mode transmission response can be obtained. The device is a one way device in that at the Bragg resonance wavelength a single mode response is retro-reflected. Light propagating in the sapphire fiber that is not at Bragg resonance is transmitted through. If another sapphire grating is coupled into the fiber further along the line with another Bragg resonance, then its light is reflected from the second grating but is retro-reflected through the first. Therefore in one length of fiber comprising tapered and sapphire links two different locations can be monitored. In this fashion, sapphire fiber grating elements can be multiplexed together into a sensor web.
In another embodiment of this invention, for lower temperature applications that are near the glass transition temperature of silica, the sapphire multimode fiber rod could be replaced with a pure silica rod. The grating that could be written could be in silica rather than sapphire if the temperatures at which the device would operate would be below the glass transition temperature of silica. In such an instance, gratings written in the silica rod would not be distorted by out diffusion of the core dopant as the fiber approaches the glass transitions temperature. If the fiber is a silica rod without dopant (ie a core) then there is no core distortion as the fiber approaches the glass transition temperature. In another embodiment of this invention, the expanded mode field propagating through the multimode fiber is guided by air-material interface with the cross-section of the fiber acting as the guiding core. The single mode reflection obtained with the taper as described above would be dependent on the effective index seen by this mode. Since the mode as it propagates through the multimode fiber has a mode field the same as the fiber diameter, its effective index is influenced by the refractive index of the medium surrounding the multimode fiber. It is therefore possible to measure the refractive index of the medium in which the multimode fiber is embedded thus fabricating a chemical sensor. This defines a new chemical sensor, for measurement of refractive index external to the sapphire rod.
It is an object of this invention to overcome the aforementioned limitations within the prior art systems for fabrication of high temperature FBG sensors by inducing refractive index modulations in optical fibers with high melting temperatures such as sapphire fiber.
It is a further object of this invention to provide a method for probing a Bragg grating structure inscribed in a multimode fiber that results in a single mode response by using tapered fiber which launches a fundamental LP01 with an expanded mode field diameter that is the same as the fiber diameter.
It is a further object of this invention to provide a method for fabrication of a single mode core in the sapphire fiber rod into which a FBG sensor can be inscribed.
It is an object of this invention to overcome the aforementioned limitations within the prior art systems for fabrication of high temperature FBG sensors by inducing refractive index modulations in optical fibers with high melting temperatures such as sapphire fiber.
It is a further object of this invention to provide a method for fabrication of a single mode core in the sapphire fiber rod into which a FBG sensor can be inscribed.
It is a further object of this invention to provide a method for fabrication of a single mode core in the sapphire rod by the inscription of a localized FBG structure along the length of the sapphire rod.